Septin Organization and Functions in Budding Yeast

Septins: Structural Insights, Functional Dynamics, and Implications in Health and Disease

Septins are a class of proteins with diverse and vital roles in cell biology. Structurally, they form hetero-oligomeric complexes and assemble into filaments, contributing to the organization of cells. These filaments act as scaffolds, aiding in processes like membrane remodeling, cytokinesis, and cell motility. Functionally, septins are essential to cell division, playing essential roles in cytokinetic furrow formation and maintaining the structural integrity of the contractile ring. They also regulate membrane trafficking and help organize intracellular organelles. In terms of physiology, septins facilitate cell migration, phagocytosis, and immune responses by maintaining membrane integrity and influencing cytoskeletal dynamics. Septin dysfunction is associated with pathophysiological conditions. Mutations in septin genes have been linked to neurodegenerative diseases, such as hereditary spastic paraplegias, underscoring their significance in neuronal function. Septins also play a role in cancer and infectious diseases, making them potential targets for therapeutic interventions. Septins serve as pivotal components of intracellular signaling networks, engaging with diverse proteins like kinases and phosphatases. By modulating the activity of these molecules, septins regulate vital cellular pathways. This integral role in signaling makes septins central to orchestrating cellular responses to environmental stimuli. This review mainly focuses on the human septins, their structural composition, regulatory functions, and implication in pathophysiological conditions underscores their importance in fundamental cellular biology. Moreover, their potential as therapeutic targets across various diseases further emphasizes their significance.

Central Role of the Actomyosin Ring in Coordinating Cytokinesis Steps in Budding Yeast

Eukaryotic cells must accurately transfer their genetic material and cellular components to their daughter cells. Initially, cells duplicate their chromosomes and subsequently segregate them toward the poles. The actomyosin ring, a crucial molecular machinery normally located in the middle of the cells and underneath the plasma membrane, then physically divides the cytoplasm and all components into two daughter cells, each ready to start a new cell cycle. This process, known as cytokinesis, is conserved throughout evolution. Defects in cytokinesis can lead to the generation of genetically unstable tetraploid cells, potentially initiating uncontrolled proliferation and cancer. This review focuses on the molecular mechanisms by which budding yeast cells build the actomyosin ring and the preceding steps involved in forming a scaffolding structure that supports the challenging structural changes throughout cytokinesis. Additionally, we describe how cells coordinate actomyosin ring contraction, plasma membrane ingression, and extracellular matrix deposition to successfully complete cytokinesis. Furthermore, the review discusses the regulatory roles of Cyclin-Dependent Kinase (Cdk1) and the Mitotic Exit Network (MEN) in ensuring the precise timing and execution of cytokinesis. Understanding these processes in yeast provides insights into the fundamental aspects of cell division and its implications for human health.

The Roles of Septins in Regulating Fission Yeast Cytokinesis

Cytokinesis is required to separate two daughter cells at the end of mitosis, and septins play crucial roles in many aspects of cytokinesis. While septins have been intensively studied in many model organisms, including the budding yeast Saccharomyces cerevisiae, septins have been relatively less characterized in the fission yeast Schizosaccharomyces pombe, which has proven to be an excellent model organism for studying fundamental cell biology. In this review, we summarize the findings of septins made in fission yeasts mainly from four aspects: the domain structure of septins, the localization of septins during the cell cycle, the roles of septins in regulating cytokinesis, and the regulatory proteins of septins.

New Insights on the Uptake and Trafficking of Coenzyme Q

Coenzyme Q (CoQ) is an essential lipid with many cellular functions, such as electron transport for cellular respiration, antioxidant protection, redox homeostasis, and ferroptosis suppression. Deficiencies in CoQ due to aging, genetic disease, or medication can be ameliorated by high-dose supplementation. As such, an understanding of the uptake and transport of CoQ may inform methods of clinical use and identify how to better treat deficiency. Here, we review what is known about the cellular uptake and intracellular distribution of CoQ from yeast, mammalian cell culture, and rodent models, as well as its absorption at the organism level. We discuss the use of these model organisms to probe the mechanisms of uptake and distribution. The literature indicates that CoQ uptake and distribution are multifaceted processes likely to have redundancies in its transport, utilizing the endomembrane system and newly identified proteins that function as lipid transporters. Impairment of the trafficking of either endogenous or exogenous CoQ exerts profound effects on metabolism and stress response. This review also highlights significant gaps in our knowledge of how CoQ is distributed within the cell and suggests future directions of research to better understand this process.

Cyclophilin A Isomerisation of Septin 2 Mediates Abscission during Cytokinesis

The isomerase activity of Cyclophilin A is important for midbody abscission during cell division, however, to date, midbody substrates remain unknown. In this study, we report that the GTP-binding protein Septin 2 interacts with Cyclophilin A. We highlight a dynamic series of Septin 2 phenotypes at the midbody, previously undescribed in human cells. Furthermore, Cyclophilin A depletion or loss of isomerase activity is sufficient to induce phenotypic Septin 2 defects at the midbody. Structural and molecular analysis reveals that Septin 2 proline 259 is important for interaction with Cyclophilin A. Moreover, an isomerisation-deficient EGFP-Septin 2 proline 259 mutant displays defective midbody localisation and undergoes impaired abscission, which is consistent with data from cells with loss of Cyclophilin A expression or activity. Collectively, these data reveal Septin 2 as a novel interacting partner and isomerase substrate of Cyclophilin A at the midbody that is required for abscission during cytokinesis in cancer cells.

Mechanistic model for nuclear migration in hyphae during mitosis

Saccharomyces cerevisiae and Candida albicans, the two well-known human pathogens, can be found in all three morphologies, i.e., yeast, pseudohyphae, and true hyphae. The cylindrical daughter-bud (germ tube) grows very long for true hyphae, and the cell cycle is delayed compared to the other two morphologies. The place of the nuclear division is specific for true hyphae determined by the position of the septin ring. However, the septin ring can localize anywhere inside the germ tube, unlike the mother-bud junction in budding yeast. Since the nucleus often migrates a long path in the hyphae, the underlying mechanism must be robust for executing mitosis in a timely manner. We explore the mechanism of nuclear migration through hyphae in light of mechanical interactions between astral microtubules and the cell cortex. We report that proper migration through constricted hyphae requires a large dynein pull applied on the astral microtubules from the hyphal cortex. This is achieved when the microtubules frequently slide along the hyphal cortex so that a large population of dyneins actively participate, pulling on them. Simulation shows timely migration when the dyneins from the mother cortex do not participate in pulling on the microtubules. These findings are robust for long migration and positioning of the nucleus in the germ tube at the septin ring.

Phosphoregulation of the septin cytoskeleton in neuronal development and disease

Septins are highly conserved GTP-binding proteins that oligomerize and form higher order structures. The septin cytoskeleton plays an important role in cellular organization, intracellular transport, and cytokinesis. Kinase-mediated phosphorylation of septins regulates various aspects of their function, localization, and dynamics. Septins are enriched in the mammalian nervous system where they contribute to neurodevelopment and neuronal function. Emerging research has implicated aberrant changes in septin cytoskeleton in several human diseases. The mechanisms through which aberrant phosphorylation by kinases contributes to septin dysfunction in neurological disorders are poorly understood and represent an important question for future research with therapeutic implications. This review summarizes the current state of knowledge of the diversity of kinases that interact with and phosphorylate mammalian septins, delineates how phosphoregulation impacts septin dynamics, and describes how aberrant septin phosphorylation contributes to neurological disorders.

Septins coordinate cell wall integrity and lipid metabolism in a sphingolipid-dependent process

Septins colocalize with membrane sterol-rich regions and facilitate recruitment of cell wall synthases during wall remodeling. We show that null mutants missing an Aspergillus nidulans core septin present in hexamers and octamers (ΔaspAcdc11, ΔaspBcdc3 or ΔaspCcdc12) are sensitive to multiple cell wall-disturbing agents that activate the cell wall integrity MAPK pathway. The null mutant missing the octamer-exclusive core septin (ΔaspDcdc10) showed similar sensitivity, but only to a single cell wall-disturbing agent and the null mutant missing the noncore septin (ΔaspE) showed only very mild sensitivity to a different single agent. Core septin mutants showed changes in wall polysaccharide composition and chitin synthase localization. Mutants missing any of the five septins resisted ergosterol-disrupting agents. Hexamer mutants showed increased sensitivity to sphingolipid-disrupting agents. Core septins mislocalized after treatment with sphingolipid-disrupting agents, but not after ergosterol-disrupting agents. Our data suggest that the core septins are involved in cell wall integrity signaling, that all five septins are involved in monitoring ergosterol metabolism, that the hexamer septins are required for sphingolipid metabolism and that septins require sphingolipids to coordinate the cell wall integrity response.

Synergistic role of nucleotides and lipids for the self-assembly of Shs1 septin oligomers

Budding yeast septins are essential for cell division and polarity. Septins assemble as palindromic linear octameric complexes. The function and ultra-structural organization of septins are finely governed by their molecular polymorphism. In particular, in budding yeast, the end subunit can stand either as Shs1 or Cdc11. We have dissected, here, for the first time, the behavior of the Shs1 protomer bound to membranes at nanometer resolution, in complex with the other septins. Using electron microscopy, we have shown that on membranes, Shs1 protomers self-assemble into rings, bundles, filaments or two-dimensional gauzes. Using a set of specific mutants we have demonstrated a synergistic role of both nucleotides and lipids for the organization and oligomerization of budding yeast septins. Besides, cryo-electron tomography assays show that vesicles are deformed by the interaction between Shs1 oligomers and lipids. The Shs1-Shs1 interface is stabilized by the presence of phosphoinositides, allowing the visualization of micrometric long filaments formed by Shs1 protomers. In addition, molecular modeling experiments have revealed a potential molecular mechanism regarding the selectivity of septin subunits for phosphoinositide lipids.

Connecting cell polarity signals to the cytokinetic machinery in yeast and metazoan cells

Polarized growth and cytokinesis are two fundamental cellular processes that exist in virtually all cell types. Mechanisms for asymmetric distribution of materials allow for cells to grow in a polarized manner. This gives rise to a variety of cell shapes seen throughout all cell types. Following polarized growth during interphase, dividing cells assemble a cytokinetic ring containing the protein machinery to constrict and separate daughter cells. Here, we discuss how cell polarity signaling pathways act on cytokinesis, with a focus on direct regulation of the contractile actomyosin ring (CAR). Recent studies have exploited phosphoproteomics to identify new connections between cell polarity kinases and CAR proteins. Existing evidence suggests that some polarity kinases guide the local organization of CAR proteins and structures while also contributing to global organization of the division plane within a cell. We provide several examples of this regulation from budding yeast, fission yeast, and metazoan cells. In some cases, kinase-substrate connections point to conserved processes in these different organisms. We point to several examples where future work can indicate the degree of conservation and divergence in the cell division process of these different organisms.

Registered report protocol: Quantitative analysis of septin Cdc10-associated proteome in Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic basidiomycetous yeast that primarily infects immunocompromised individuals. C. neoformans can thrive during infections due to its three main virulence-related characteristics: the ability to grow at host temperature (37°C), formation of carbohydrate capsule, and its ability to produce melanin. C. neoformans strains lacking septin proteins Cdc3 or Cdc12 are viable at 25°C; however, they fail to proliferate at 37°C and are avirulent in the murine model of infection. The basis of septin contribution to growth at host temperature remains unknown. Septins are a family of conserved filament-forming GTPases with roles in cytokinesis and morphogenesis. In the model organism Saccharomyces cerevisiae septins are essential. S. cerevisiae septins form a higher order complex at the mother-bud neck to scaffold over 80 proteins, including those involved in cell wall organization, cell polarity, and cell cycle control. In C. neoformans, septins also form a complex at the mother-bud neck but the septin interacting proteome in this species remains largely unknown. Moreover, it remains possible that septins play other roles important for high temperature stress that are independent of their established role in cytokinesis. Therefore, we propose to perform a global analysis of septin Cdc10 binding partners in C. neoformans, including those that are specific to high temperature stress. This analysis will shed light on the underlying mechanism of survival of this pathogenic yeast during infection and can potentially lead to the discovery of novel drug targets.

Role of combined cell membrane and wall mechanical properties regulated by polarity signals in cell budding

Budding yeast, Saccharomyces cerevisiae, serves as a prime biological model to study mechanisms underlying asymmetric growth. Previous studies have shown that prior to bud emergence, polarization of a conserved small GTPase Cdc42 must be established on the cell membrane of a budding yeast. Additionally, such polarization contributes to the delivery of cell wall remodeling enzymes and hydrolase from cytosol through the membrane, to change the mechanical properties of the cell wall. This leads to the hypothesis that Cdc42 and its associated proteins at least indirectly regulate cell surface mechanical properties. However, how the surface mechanical properties in the emerging bud are changed and whether such change is important are not well understood. To test several hypothesised mechanisms, a novel three-dimensional coarse-grained particle-based model has been developed which describes inhomogeneous mechanical properties of the cell surface. Model simulations predict alternation of the levels of stretching and bending stiffness of the cell surface in the bud region by the polarized Cdc42 signals is essential for initiating bud formation. Model simulations also suggest that bud shape depends strongly on the distribution of the polarized signaling molecules while the neck width of the emerging bud is strongly impacted by the mechanical properties of the chitin and septin rings. Moreover, the temporal change of the bud mechanical properties is shown to affect the symmetry of the bud shape. The 3D model of asymmetric cell growth can also be used for studying viral budding and other vegetative reproduction processes performed via budding, as well as detailed studies of cell growth.

Genes and lipids that impact uptake and assimilation of exogenous coenzyme Q in Saccharomyces cerevisiae

Coenzyme Q (CoQ) is an essential player in the respiratory electron transport chain and is the only lipid-soluble antioxidant synthesized endogenously in mammalian and yeast cells. In humans, genetic mutations, pathologies, certain medical treatments, and aging, result in CoQ deficiencies, which are linked to mitochondrial, cardiovascular, and neurodegenerative diseases. The only strategy available for these patients is CoQ supplementation. CoQ supplements benefit a small subset of patients, but the poor solubility of CoQ greatly limits treatment efficacy. Consequently, the efficient delivery of CoQ to the mitochondria and restoration of respiratory function remains a major challenge. A better understanding of CoQ uptake and mitochondrial delivery is crucial to make this molecule a more efficient and effective therapeutic tool. In this study, we investigated the mechanism of CoQ uptake and distribution using the yeast Saccharomyces cerevisiae as a model organism. The addition of exogenous CoQ was tested for the ability to restore growth on non-fermentable medium in several strains that lack CoQ synthesis (coq mutants). Surprisingly, we discovered that the presence of CoQ biosynthetic intermediates impairs assimilation of CoQ into a functional respiratory chain in yeast cells. Moreover, a screen of 40 gene deletions considered to be candidates to prevent exogenous CoQ from rescuing growth of the CoQ-less coq2Δ mutant, identified six novel genes (CDC10, RTS1, RVS161, RVS167, VPS1, and NAT3) as necessary for efficient trafficking of CoQ to mitochondria. The proteins encoded by these genes represent essential steps in the pathways responsible for transport of exogenously supplied CoQ to its functional sites in the cell, and definitively associate CoQ distribution with endocytosis and intracellular vesicular trafficking pathways conserved from yeast to human cells.

Yeast as a Model to Understand Actin-Mediated Cellular Functions in Mammals-Illustrated with Four Actin Cytoskeleton Proteins

The budding yeast Saccharomyces cerevisiae has an actin cytoskeleton that comprises a set of protein components analogous to those found in the actin cytoskeletons of higher eukaryotes. Furthermore, the actin cytoskeletons of S. cerevisiae and of higher eukaryotes have some similar physiological roles. The genetic tractability of budding yeast and the availability of a stable haploid cell type facilitates the application of molecular genetic approaches to assign functions to the various actin cytoskeleton components. This has provided information that is in general complementary to that provided by studies of the equivalent proteins of higher eukaryotes and hence has enabled a more complete view of the role of these proteins. Several human functional homologues of yeast actin effectors are implicated in diseases. A better understanding of the molecular mechanisms underpinning the functions of these proteins is critical to develop improved therapeutic strategies. In this article we chose as examples four evolutionarily conserved proteins that associate with the actin cytoskeleton: (1) yeast Hof1p/mammalian PSTPIP1, (2) yeast Rvs167p/mammalian BIN1, (3) yeast eEF1A/eEF1A1 and eEF1A2 and (4) yeast Yih1p/mammalian IMPACT. We compare the knowledge on the functions of these actin cytoskeleton-associated proteins that has arisen from studies of their homologues in yeast with information that has been obtained from in vivo studies using live animals or in vitro studies using cultured animal cell lines.

Cytokinesis in Eukaryotic Cells: The Furrow Complexity at a Glance

The duplication cycle is the fascinating process that, starting from a cell, results in the formation of two daughter cells and it is essential for life. Cytokinesis is the final step of the cell cycle, it is a very complex phase, and is a concert of forces, remodeling, trafficking, and cell signaling. All of the steps of cell division must be properly coordinated with each other to faithfully segregate the genetic material and this task is fundamental for generating viable cells. Given the importance of this process, molecular pathways and proteins that are involved in cytokinesis are conserved from yeast to humans. In this review, we describe symmetric and asymmetric cell division in animal cell and in a model organism, budding yeast. In addition, we illustrate the surveillance mechanisms that ensure a proper cell division and discuss the connections with normal cell proliferation and organs development and with the occurrence of human diseases.

A Fungal Arrestin Protein Contributes to Cell Cycle Progression and Pathogenesis

To survive under unpredictable conditions, all organisms must adapt to stressors by regulating adaptive cellular responses. Arrestin proteins are conserved regulators of adaptive cellular responses in eukaryotes. Studies that have been limited to mammals and model fungi have demonstrated that the disruption of arrestin-regulated pathways is detrimental for viability. The human fungal pathogen Cryptococcus neoformans causes more than 180,000 infection-related deaths annually, especially among immunocompromised patients. In addition to being genetically tractable, C. neoformans has a small arrestin family of four members, lending itself to a comprehensive characterization of its arrestin family. This study serves as a functional analysis of arrestins in a pathogen, particularly in the context of fungal fitness and virulence. We investigate the functions of one arrestin protein, Ali1, and define its novel contributions to cytokinesis. We additionally explore the virulence contributions of the C. neoformans arrestin family and find that they contribute to disease establishment and progression.

Arrestins, a structurally specialized and functionally diverse group of proteins, are central regulators of adaptive cellular responses in eukaryotes. Previous studies on fungal arrestins have demonstrated their capacity to modulate diverse cellular processes through their adaptor functions, facilitating the localization and function of other proteins. However, the mechanisms by which arrestin-regulated processes are involved in fungal virulence remain unexplored. We have identified a small family of four arrestins, Ali1, Ali2, Ali3, and Ali4, in the human fungal pathogen Cryptococcus neoformans. Using complementary microscopy, proteomic, and reverse genetics techniques, we have defined a role for Ali1 as a novel contributor to cytokinesis, a fundamental cell cycle-associated process. We observed that Ali1 strongly interacts with proteins involved in lipid synthesis, and that ali1Δ mutant phenotypes are rescued by supplementation with lipid precursors that are used to build cellular membranes. From these data, we hypothesize that Ali1 contributes to cytokinesis by serving as an adaptor protein, facilitating the localization of enzymes that modify the plasma membrane during cell division, specifically the fatty acid synthases Fas1 and Fas2. Finally, we assessed the contributions of the C. neoformans arrestin family to virulence to better understand the mechanisms by which arrestin-regulated adaptive cellular responses influence fungal infection. We observed that the C. neoformans arrestin family contributes to virulence, and that the individual arrestin proteins likely fulfill distinct functions that are important for disease progression.

Distinct segregation patterns of yeast cell-peripheral proteins uncovered by a method for protein segregatome analysis

Protein segregation contributes to various cellular processes such as polarization, differentiation, and aging. However, the difficulty in global determination of protein segregation hampers our understanding of its mechanisms and physiological roles. Here, by developing a quantitative proteomics technique, we globally monitored segregation of preexisting and newly synthesized proteins during cell division of budding yeast, and identified crucial domains that determine the segregation of cell-peripheral proteins. Remarkably, the proteomic and subsequent microscopic analyses demonstrated that the flow through the bud neck of the proteins that harbor both endoplasmic reticulum (ER) membrane-spanning and plasma membrane (PM)-binding domains is not restricted by the previously suggested ER membrane or PM diffusion barriers but by septin-mediated partitioning of the PM-associated ER (pmaER). Furthermore, the proteomic analysis revealed that although the PM-spanning t-SNARE Sso2 was retained in mother cells, its paralog Sso1 unexpectedly showed symmetric localization. We found that the transport of Sso1 to buds was required for enhancement of polarized cell growth and resistance to cell-wall stress. Taken together, these data resolve long-standing questions about septin-mediated compartmentalization of the cell periphery, and provide new mechanistic insights into the segregation of cell-periphery proteins and their cellular functions.

Molecular mechanisms of contractile-ring constriction and membrane trafficking in cytokinesis

In this review, we discuss the molecular mechanisms of cytokinesis from plants to humans, with a focus on contribution of membrane trafficking to cytokinesis. Selection of the division site in fungi, metazoans, and plants is reviewed, as well as the assembly and constriction of a contractile ring in fungi and metazoans. We also provide an introduction to exocytosis and endocytosis, and discuss how they contribute to successful cytokinesis in eukaryotic cells. The conservation in the coordination of membrane deposition and cytoskeleton during cytokinesis in fungi, metazoans, and plants is highlighted.

Dissecting the nucleotide binding properties of the septins from S. cerevisiae

Septins are a conserved family of guanosine triphosphate (GTP)-binding proteins that assemble into an ordered array of filaments at the mother bud neck in Saccharomyces cerevisiae cells. They are present in all higher eukaryotes except plants. Septins belong structurally to the P-Loop nucleoside triphosphatase (NTPases) like Rab and Ras. However, unlike other small guanosine triphosphatase (GTPases) septins are supposed to act as scaffolds rather than signalling mediators. This is why they are considered as the fourth class of cytoskeletal proteins. It is assumed that septins fulfil their functions independently of the bound nucleotide. The role of guanosine diphosphosphate (GDP) and GTP binding and subsequent hydrolysis was controversial debated in the last couple of years. Lack of crystal structures of yeast septin subunits or rods and difficulties to isolate single monomeric septin subunits often hindered the correlation of results obtained from in vivo studies with biochemical data. Recently, nucleotide binding and hydrolysis was connected to the formation of septin rods from its subunits. However, the evidence was only indirectly obtained through the use of septin mutants in the context of intact cells. We provide here mechanistic insight into the nucleotide binding of the yeast septins by in vitro assays using purified septin rods and building blocks, thereby adding further insights to the already available models on septin filament formation.

A humanized yeast-based toolkit for monitoring phosphatidylinositol 3-kinase activity at both single cell and population levels

Phosphatidylinositol 3-kinase (PI3K) is a key regulator of phosphoinositide-dependent signaling in mammalian cells and its dysfunction is related to multiple syndromes, including cancer. By heterologous expression in Saccharomyces cerevisiae, we have developed a humanized yeast system as a tool for functional studies on higher eukaryotic PI3K. Here we restrict PI3K activity in yeast to specific plasma membrane (PM) microdomains by fusing the p110α PI3K catalytic subunit to either a septin or an eisosome component. We engineered a Dual Reporter for PI3K (DRAPIK), useful to monitor activity on cellular membranes in vivo at a single-cell level, by simultaneous PM staining of the enzyme substrate (PtdIns4,5P₂) with GFP and its product (PtdIns3,4,5P₃) with mCherry. We also developed a sensitive FLUorescence by PI3K Inhibition (FLUPI) assay based on a GFP transcriptional reporter that is turned off by PI3K activity. This reporter system proved useful to monitor PI3K inhibition in vivo by active compounds. Such novel tools were used to study the performance of yeast PM microdomain-directed PI3K. Our results show that tethering heterologous PI3K to discrete PM domains potentiates its activity on PtdIns4,5P₂ but different locations display distinct effects on yeast growth and endocytosis.

Persistent actin depolarization caused by ethanol induces the formation of multiple small cortical septin rings in yeast

Short-term exposure to severe ethanol stress has adverse effects on yeast cells. However, limited information is available on the effects of long-term exposure to severe ethanol stress. In this study, we examined the effects of a long-term treatment with a high ethanol concentration [10% (v/v)] on yeast morphology. We found that long-term severe ethanol stress induced the continuous depolarization of the actin cytoskeleton and hypertrophy in yeast cells, accompanied by the aberrant localization of septins, which formed multiple small cortical rings (MSCRs). The formation of MSCRs was also induced by the continuous depolarization of the actin cytoskeleton caused by a treatment with latrunculin-A, an effective inhibitor of actin polymerization. Unlike the formation of conventional septin rings, the formation of MSCRs did not require Cdc42 and its effectors, Gic1, Gic2 and Cla4. These results provide novel insights into the effects of persistent actin depolarization caused by long-term exposure to severe ethanol stress on yeast cytomorphology.

Septin localization and function during autophagy

Autophagy is a vital conserved recycling process where eukaryotic cells remove unwanted proteins and organelles via lysosomal degradation and in turn, generate nutrients for the cells. The special feature of autophagy process is the formation of double-membrane vesicles called autophagosomes that engulf cellular cargo and deliver them to the vacuole or lysosomes for degradation. Inspite of more than 40 AuTophaGy (ATG) proteins and several organelles as known membrane source, autophagosome biogenesis is not entirely understood. We recently have discovered that septins contribute to autophagosome biogenesis. Septins are GTP-binding proteins, usually localized at the bud neck region and are involved in cytokinesis. Here, we show that during autophagy prevalent conditions, septins traffic between different cellular compartments such as Golgi, mitochondria, endosomes, plasma membrane, and vacuolar membranes.

Septins are involved at the early stages of macroautophagy in S. cerevisiae

Autophagy is a conserved cellular degradation pathway wherein double-membrane vesicles called autophagosomes capture long-lived proteins, and damaged or superfluous organelles, and deliver them to the lysosome for degradation. Septins are conserved GTP-binding proteins involved in many cellular processes, including phagocytosis and the autophagy of intracellular bacteria, but no role in general autophagy was known. In budding yeast, septins polymerize into ring-shaped arrays of filaments required for cytokinesis. In an unbiased genetic screen and in subsequent targeted analysis, we found autophagy defects in septin mutants. Upon autophagy induction, pre-assembled septin complexes relocalized to the pre-autophagosomal structure (PAS) where they formed non-canonical septin rings at PAS. Septins also colocalized with autophagosomes, where they physically interacted with the autophagy proteins Atg8 and Atg9. When autophagosome degradation was blocked in septin-mutant cells, fewer autophagic structures accumulated, and an autophagy mutant defective in early stages of autophagosome biogenesis (atg1Δ), displayed decreased septin localization to the PAS. Our findings support a role for septins in the early stages of budding yeast autophagy, during autophagosome formation.This article has an associated First Person interview with the first author of the paper.

The step-wise pathway of septin hetero-octamer assembly in budding yeast

Septin proteins bind guanine nucleotides and form rod-shaped hetero-oligomers. Cells choose from a variety of available septins to assemble distinct hetero-oligomers, but the underlying mechanism was unknown. Using a new in vivo assay, we find that a stepwise assembly pathway produces the two species of budding yeast septin hetero-octamers: Cdc11/Shs1–Cdc12–Cdc3–Cdc10–Cdc10–Cdc3–Cdc12–Cdc11/Shs1. Rapid GTP hydrolysis by monomeric Cdc10 drives assembly of the core Cdc10 homodimer. The extended Cdc3 N terminus autoinhibits Cdc3 association with Cdc10 homodimers until prior Cdc3–Cdc12 interaction. Slow hydrolysis by monomeric Cdc12 and specific affinity of Cdc11 for transient Cdc12•GTP drive assembly of distinct trimers, Cdc11–Cdc12–Cdc3 or Shs1–Cdc12–Cdc3. Decreasing the cytosolic GTP:GDP ratio increases the incorporation of Shs1 vs Cdc11, which alters the curvature of filamentous septin rings. Our findings explain how GTP hydrolysis controls septin assembly, and uncover mechanisms by which cells construct defined septin complexes.

DOI:http://dx.doi.org/10.7554/eLife.23689.001